1. Field of the Invention
The present invention relates to positron emission tomography (PET). More particularly, the present invention relates to an apparatus for providing geometrically configurable mechanical support PET detector arrays.
2. Description of Related Art
Detectors employed for PET scanning (imaging) are relatively small compared to other detectors used for detecting photons. For instance, PET detectors are about 200 times smaller than the large detectors for high-energy physics and require identification of only one type of particles, the photons. The task of capturing and identifying the particles is relatively easier than before: one type of particle instead of five and on a detector 200 times smaller.
The use of positron emissions for medical imaging has been well documented from the early 1950's, see “A History of Positron Imagining,” Brownell, Gordon, presented on Oct. 15, 1999, Massachusetts General Hospital and available at http://neurosurgery.mgh.harvard.edu/docs/PEThistory.pdf, which is incorporated herein by reference in its entirety. PET imaging has advantages over other types of imaging procedures. Generally, PET scanning provides a procedure for imaging the chemical functionality of body organs rather than imaging only their physical structure, as is commonly available with other types of imaging procedures such as X-ray, computerized tomography (CT), or magnetic resonance imaging (MRI). PET scanned images allow a physician to examine the functionality of the heart, brain, and other organs as well as to diagnose disease groups which cause changes in the cells of a body organ or in the manner they grow, change, and/or multiply out of control, such as cancers.
Other applications for detecting particles (photons, electrons, hadron, muon and jets) are well known, such as with regard to experiments in high energy physics. While particle detection in high energy physics and medical imaging have some common ground, differences between the disciplines are sticking. One distinction between the usages is that the detectors used in medical imaging are approximately 200 times smaller than the larger detectors employed in high-energy physics applications, and what is more, medical imaging PET applications require the identification of only a single type of particle, the photon.
Typically, prior art PET devices require the injection into the patient's body of a radiation dose that is 10 to 20 times the maximum radiation dose recommended by the International Commission on Radiological Protection (ICRP). This amount is necessary because, at best, prior art PET devices detect only two photons out of 10,000 emitted in the patients' body. Currently the largest manufacturers of PET (General Electric Company and Siemens AG (ADR)) which command in excess of 90% of the world market, are manufacturing two different PET (PET/CT) systems with very similar performance and are selling them at very similar prices. However, although the price and performance of the systems from the different manufactures are comparable, one manufacturer's system (Siemens) uses nearly ideal crystal detectors, while in contrast, the other manufacturer's system (General Electric) uses cheaper, lower quality crystal detectors with slower decay time. Consequently, the manufacturer using the cheaper, lower cost detectors, expends on the order of only 10% the price of the ideal crystals used in their competitor's systems. Thus, the question arises: how it could be that even though one manufacturer uses crystals detectors that are ten times more expensive that the other manufacturer, the price and performance of the two PET systems from the different manufacturers are very comparable.
Anecdotally, the present inventor has analyzed the progress of the most significant PET improvements made in the most recent 17 years, see “400+times improved PET efficiency for lower-dose radiation, lower-cost cancer screening,” 3D-Computing, Jun. 30, 2001, ISBN: 0970289707, which is incorporated herein by reference in its entity. The improved PET device is also taught by the present inventor in co-pending U.S. Non-Provisional patent application Ser. No. 10/250,791, entitled “Method And Apparatus For Anatomical And Functional Medical Imaging,” relating to and claiming priority from PCT/US01/15671, filed May 15, 2001 which relates to and claims priority from U.S. Provisional Patent Application No. 60/204,900 filed May 16, 2000, U.S. Provisional Patent Application No. 60/215,667 filed Jun. 30, 2000, U.S. Provisional Patent Application No. 60/239,543 filed Oct. 10, 2000, U.S. Provisional Patent Application No. 60/250,615 filed Nov. 30, 2000, U.S. Provisional Patent Application No. 60/258,204 filed Dec. 22, 2000 and U.S. Provisional Patent Application No. 60/261,387 filed Jan. 15, 2000 which are each incorporated herein by reference in their entirety.
Problems inherent in the prior art PET devices include low device efficiency, poor image quality due to, for instance, low spatial resolution, long examination times, and high dosages of radiation to the patient. These shortcomings result in high examinations costs to the patient, prolonged payback of capital and unsuitability of the current PET technology to adapt to well-patient procedures. These shortcomings are described in greater specificity U.S. patent application Ser. No. 10/376,024 filed on Feb. 26, 2003 titled “Method and Apparatus for Determining Depth of Interactions in a Detector for Three-Dimensional Complete Body Scanning” and which is incorporated herein by reference in its entirety.
FIG. 1 is a simplified diagram of a PET device as known in the prior art. Essentially PET scanner 100 provides a plurality of scintillation detector assemblies arranged in a cylindrical geometric configuration as is well known in the prior art. Each detector assembly comprises a crystal 112, and at least one a light amplifier per detector. Crystal 112 might be any type which interacts with a photon so as to produce a scintillation, or rapid flash of light in the interior lattice structure of the crystal. Typically, crystal 112 is optically coupled to one or more optical amplifiers which have a detector integrated therein. Thus, as a practical matter amplifiers 114 may be Photomultipliers (PMTs), Avalanche Photodiodes (APDs) or some other type of light emitting diode, however each amplifier-detector combination will have a signal output (a channel) for outputting the amplified signal to the processing electronics.
As mentioned above, detector array 110 is geometrically configured as an open ended cylinder, having ingress opening 102 and egress opening 104 of sufficient diameter for accepting the cross-sectional diameter of a patient's body. As compared to the height of a human body, the total detector length of array 110 is rather small, typically on the order of 5.9 in. to 9.8 in. (15 cm.-25 cm.). This is known as the field of view (FOV) of the detector array. The reason for the prior art PET devices having very small FOVs is because, among other reasons, the capital expense in the detectors. Even if a PET were configured with a larger FOV, the resulting device would not overcome the shortcoming of the prior art because prior art technologies do not fully exploit the double photon emission phenomenon. Moreover, current PET devices utilized electronics that saturate, even at relatively modest photon capture rates. Thus, any increase in the FOV over which the electronics can process the additional photons are wasted. By way of example, typically prior art PET devices capture on the order of two photons for every 10,000 photons emitted from the patient's body. Thus, it takes approximately 55 minutes to scan 70 cm FOV. Clearly utilizing only two out of every 50,000 photons available drastically reduces the data quality and lower resolution images are the result.
Typically the patient is conveyed along the interior of cylindrical detector array 110 for the device to effectively scan the patient's body. Turning to FIG. 2, a cross-sectional view detector array 110 of PET 100 is depicted. Also show is patient 220, who is oriented substantially coaxial with the FOV of detector array 110. Notice that the cross-sectional view of array 110 depicts the detectors as being configured in a near-perfect circle. This configuration is necessary in order to lessen the effects of parallax errors. A error results from assuming that photons strike the detector at 90 degrees to its face. It is expected that photons enter the crystal following a path which is perpendicular to the face of the crystal and parallel with the length of the detector, i.e., straight into one detector only. When a photon enters the crystal at 90 degrees, its X-Y position can be easily calculated from the detectors which perceive the scintillation effect in the crystal, the X-Y position through a centroid calculation. The depth at which the photon interacts with the crystal is unimportant in this case where the photon penetrates the crystal perpendicular to the face, because it will interact somewhere along a line in oriented in the Z direction formed by the intersection of an X plane and a Y plane, i.e. the line of response (LOR) is found perpendicular to the X-Y planes. This presumes that all lines of response between coincidental pairs of detectors intersect the center point of the barrel.
Given the already low efficiency of the current PET devices, configuring the cross-section of detector array 10 in a circle gives better results because some parallax errors are avoided because a largest proportion of the photons must enter the crystal at 90 degrees from the crystal's face.
This configuration has several undesirable consequences. The first is that array 10 is permanently configured as a circle. Claustrophobic patients must be transported through the interior of the PET for optimal results. Recall that for imaging, a FOV of a mere 27 inches requires that the patient remaining motionless for 55 minutes, which is difficult for patients not suffering from claustrophobia and nearly impossible for those who do suffer from claustrophobia.
Furthermore, notice from FIG. 2, that while the radial displacement of the detectors does reduce parallax error, it cannot be eliminated altogether. The circularly configuration necessarily orients all detectors toward the center point of the circle and therefore handles only photons which are generated along those paths, shown in the figure as photon paths 230. Photons generated in the extremities of the patients body, or those not traveling in pathways 230 may still result in parallax error which will reduce the image quality.
Aside from the cost of large diameter arrays, most facilities simply do not have the vertical clearance for supporting a detector array diameter sufficient for scanning larger individuals. Thus, in addition to those patients suffering from claustrophobia, patients with larger frames and/or obese patients can not be accommodated with current PET technology.
As a final matter, the PET scanning process itself dictates the use of detector array oriented in a circular configuration because the patient's body is transported across the FOV of the detectors rather than being scanned while stationary. Since the cross-section of a patient's body patient continually changes with respect to the array as it passes through, the prior art simply have no alternative but to compromise on circular array configuration for handling any cross-sectional shape. Even with respect to parts of the body where a circular detector array would be most optimal, such as the patient's head, the detector array is situated for the maximum cross-sectional area of the patient, i.e., the torso, and therefore cannot give the best results in more narrow regions of the body.
What is needed is a means for increasing the accuracy and efficiencies of PET devices enabling caregivers to more accurately diagnose aliments related to the functionality of body organs and not just inferences from the structure of the organs. Also, what is needed is a more flexible device which will accommodate more patients, those who suffer from claustrophobia, obesity or who are simply larger individuals. Also what is needed is a geometrical configurable PET for use in research and academia.